This invention relates to the art of wireless communication, and more particularly, to a system for efficient paging of wireless terminals.
Prior art wireless systems employ a technique known as sleeping, which is coupled with a technique known as paging, to conserve resources, e.g., power and/or bandwidth, when the wireless terminal is not expected to be in its active communicating state, e.g., on call, for some time. Often the time that a wireless terminal is determined to not be expected to be in its active communicating state is a function of its prior activity, e.g., when the wireless terminal has not been communicating actively for a prescribed time period. When the wireless terminal is not expected to be in its active communicating state it goes into xe2x80x9csleepxe2x80x9d mode, a mode in which both the receiver and transmitter of the wireless terminal consume significantly less power. For example, in some systems, both receiver and transmitter are turned off during this sleep time.
A wireless terminal prior to going to sleep is assigned a specific paging channel land time. The paging channel is at least a logical channel, although it may be a physical channel, that is used to communicate pages, i.e., paging messages. At the assigned time, the wireless terminal xe2x80x9cwakes upxe2x80x9d, tunes its receiver to the assigned paging channel, and listens to see if there are any pages for it, which may simply be the transmission of an identifier for the wireless terminal. If there is a page, and the page does not contain the entirety of the message to be communicated to the wireless terminal, then the wireless terminal may tune to a channel, which may be known in advance to the wireless terminal or may be specified as part of the page, and conduct the appropriate information exchange to receive the message.
Notwithstanding that they may be in sleep mode, wireless terminals may move. As a result, their location may become such that a better signal may be obtained from a base station other than the one with which they communicated prior to going to sleep. When changing base stations, a wireless terminal would be expected to conduct a xe2x80x9chandoffxe2x80x9d whereby the new base station is contacted and arrangements are made for it to serve the wireless terminal. However, such handoffs are power consuming and use system resources that could otherwise be used to carry data traffic. To avoid these penalties, prior art systems have defined xe2x80x9cpaging areasxe2x80x9d in which a sleeping wireless terminal need not conduct a handoff. Instead, all of the base stations within the paging area will broadcast any pages for the wireless terminal. The wireless terminal then needs only to tune its receiver to the base station having the best signal for communicating that is in the paging area. A central controller keeps track of which paging area each wireless terminal is in.
The prior art base stations also broadcast the identity of the paging area to which they belong. When a wireless terminal leaves a paging area, it can detect that it has left by noting the difference in the paging area identity it is now receiving from that which it was previously receiving. It will then conduct a handoff to the base station having the best signal for communicating with and will arrange to join the new paging area. The central controller will then route future messages for the wireless terminal to its new paging area.
Disadvantageously, prior art wireless systems need to know a priori the map of the paging areas. Typically such information has a high cost because it is developed during the course of system engineering or planning before constructing the network. Also disadvantageously, because of the fixed paging area boundaries, base stations on the edge of a given paging area are heavily loaded by the handoffs of page mode wireless terminals, and thus their capacity for normal data traffic is reduced.
We have recognized that the cost of wireless systems can be reduced significantly, and their performance increased, through the use of dynamic, wireless-terminal-based paging area assignment. Although it is possible to use the technique of the instant invention when the network architecture is known, to further reduce costs, preferably use is made of a wireless terminal controlled handoff process by which at least portions of the xe2x80x9cmapxe2x80x9d, i.e., the pattern of neighboring base stations, for use as a paging area, is discovered and updated. One such self-discovering network arrangement is disclosed in our copending United States Patent Application Serial No. (case Davies-Vanderveen 1-5), which is incorporated by reference as if set forth entirely herein. In such an arrangement, it takes at least one handoff for each such base station neighbor to discover its entire local map. Once the map of the network architecture is known, paging areas aligned with the wireless terminal""s initial base station may be dynamically created, in accordance with an aspect of the invention. Advantageously, because there are no fixed paging area boundaries, paging handoffs may be spread relatively uniformly across base stations, thus avoiding the base station loading problem associated with fixed paging area boundaries.
More specifically, as part of the handoff process, the wireless terminal tells the new base station to which it is handing off the call the identity of the previous base station from which control is being handed off. If the new base station does not have a valid record for the previous base station, it forms a trust relationship with the previous base station and creates a record for it, thus identifying it as an immediate neighbor. Additionally, both base stations exchange lists of other base stations known to them, preferably with the constraint that these other base stations must be within a prescribed number of handoffs less one of the base station supplying the list. For example, a distance of two handoffs means that in travelling from base station A to base station C, a wireless terminal would hand off from A to B and then from B to C, thus performing a total of two handoffs. Thus the base station receiving the list will know of all the base stations within the prescribed number of handoffs of the receiving base station of which the base station supplying the list is aware.
When a wireless terminal first starts its session after power up, it forms a connection with its first base station. This base station then passes to the wireless terminal a list of all the base stations that are known to the first base station and are within the prescribed number of handoffs of the first base station. The wireless terminal then uses this list to define its own xe2x80x9cpersonalxe2x80x9d paging area. Thus, the prescribed number corresponds, loosely speaking, to a paging area radius for the wireless terminal""s personal paging area, measured in units of handoffs. Each time the wireless terminal emerges from a sleep state, the wireless terminal listens for the base station having the best signal for communicating and compares its identification against the list of base stations in its personal paging area. If the base station having the best signal for communicating is on the list, then the wireless terminal knows that any paging messages for it will be automatically broadcast by the base station having the best signal for communicating. If the base station is not on the list, then the wireless terminal must conduct a handoff to this base station, and so it obtains from this base station a new personal paging area centered on that base station. Note that such handoffs are spread randomly throughout the whole system as the initial locations that wireless terminals start their sessions are random, as are their motion within the wireless communication coverage area.
The base station most recently engaged in a handoff with the wireless terminal, or the one with which the session was started with if handoffs have not yet been performed, serves as a paging agent for the wireless terminal. Messages originating elsewhere in the network are forwarded to the paging agent for delivery to the wireless terminal. The paging agent then instructs all known base stations within prescribed number of handoffs of it to page the wireless terminal. The wireless terminal, on hearing the page, will form a connection with the base station having the best signal for communicating. The base station having the best signal for communicating notifies the paging agent that it has established a connection with the wireless terminal, which caves paging mode and becomes active.